Overspeed control apparatus



March 17, 1959 M. M. HoBBs 2,877,783

ovERsPEED CONTROL APPARATUS Filed Sept. 4, 1956 :s sheets-sheet 1 March 17, 1959 vM. M. HOBBS ovERsPEED CONTROL APPARATUS 3 Sheets-Sheet 2 Filed Sept. 4, 1956 mvENToR MILTON M. Hoses BWYW ATTORNEY March 1 7, 1959V M. M. HoBBs 2,877,783

ovERsPEED CONTROL APPARATUS I Filedsept. 4, 1956 's sheets-sheet s THROTTLE III I I III/l I vl Y INVENToR MILTON M.HoeBs HIGH PRESSURE LIQUID ATTORNEY OVERSPEEI) CONTROL APPARATUS Milton M. Hobbs, Broomall, Pa., assiguor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 4, 1956, Serial No. 607,937

9 Claims. (Cl. 137-31) This invention relates to an overspeed trip mechanism, more particularly to an automatic overspeed trip mechanism for a turbine or other rotating machine and has for an object to provide a more reliable and otherwise improved mechanism of this type.

The present application is a continuation in part of application S. N. 514,573, tiled June 10, 1955, for Overspeed Control Apparatus, and assigned to the assignee of the present application, now abandoned.

In presently known overspeed trip devices utilizing a spring loaded weight, carried by the shaft of the machine to be controlled, and movable radially outwardly to actuate a trip lever under overspeed conditions, friction may prevent the weight from actuating the trip lever at the preselected speed. Also, at high shaft speeds there is the tendency for the weight to move outwardly with a force sufiiciently great to break the trip lever. In addition to the above, the weight is usually disposed in the end portion of the shaft, thus occupying the region desirable for an axial bore needed for test instruments (strain gage wires, etc.).

In view of the above, it is a further object of the invention to provide an overspeed trip mechanism which permits free access to the end of the machine shaft whose speed is being controlled.

Another object is to provide an overspeed trip mechanism which will fail safe, i. e. will be eiective to shut down the machine in the event of failure of any portion of the hydraulic system.

A more specic object of the invention is to provide an overspeed sensing device which eiects a reduction in p pressure of the controlling iluid as the speed of the machine increases beyond the preselected speed.

Yet another object of the invention is to provide an overspeed trip mechanism of the above type in which the tripping speed adjusting means may be disposed remotely from the machine.

Briey, the invention resides in providing a novel speed sensing device which is attached to the shaft of the engine and has a rotary member provided with an input impeller and an output impeller disposed in registry with an input port and an output port on an encompassing stationary member. A passage acting ,as a communication between the input impeller and theoutput impeller is provided. The input port is connectedto a source of liquid under constant uniform pressure while the output port is connected to a pressure responsive piston valve arrangement controlling the throttle to the engine and acting in response to reduction in liquid pressure to close the throttle. A means is provided in the passage communicating between the input impeller and the output impeller for restricting the flow of liquid through the passage when. a preselected speed is attained by the shaft.

In operation, under normal speed conditions the liquid will ow through both impellers to the piston valve, maintaining it in throttle opening position. when the prescribed speed is exceeded, the iiow of liquid through the input impeller is terminated. The liquid However,

' Pater which is in the pressure responsive piston valve is per'-I mitted to escape through a leakage path and thereby the pressure of the liquid acting on the piston valve is reduced to a value at which the valve s eective to close the throttle, thereby causing the engine speed to be reduced.

The overspeed trip mechanism further includes a speed regulating device including a pressure regulating valve which is manually adjustable to vary the pressure of the liquid supplied to the input port of the speed sensing device.

The above and other objects areeffected by the invention as will be apparent from the following description taken in connection with the accompanying drawings, forming a part of this application, in which:

Fig. 1 is a schematic sectional view of a device constructed in accordance with the invention; j

Fig. 2 is a cross-section taken on line II-II of Fig. 1;

Fig. 3 is a sectional view showing a modified form of the invention;

Fig. 4 is a cross-section taken on line'IV-IV of Fig..3;

Fig. 5 is a schematic sectional view, similar to Fig. 1, but showing another modification of the invention;

Fig. 6 is a cross-section taken on line VI-VI of Fig. 5; and Y Fig. 7 is a fragmentary cross-section taken on line VII--VII of Fig. 5.

Referring to Figs. l and 2 the overspeed trip mechanism comprises a hydraulic overspeed sensing devicev10 carried on the end of a shaft 12 of an engine, for example, a steam turbine.

The overspeed sensing device 10 is provided with an inner cylindrical member 14 which is fastened to the engine shaft in any desirable manner, for example, by a threaded portion 15, so that it is rotatable therewith. A stationary sleeve 16 disposed in telescoping relation .with

the rotatable member 14 is provided with a pair of ports,

namely, an input port 1'7 and an output port 18 preferably having an annular shape and disposed in alignment with a series of input apertures 19 and a series of output apertures 2li, respectively, formed in the periphery of the rotatable member 14. The set of input apertures 19 constitutes an input impeller while the set of output apertures constitutes an output impeller. The rotatable member is further provided with an annular passageway 22 extending from the input impeller 19 to the output impeller 20 and partially detined by an axially disposed inner sleeve member 23 so that fluid entering the apertures 19 may flow through the passageway 22 to the output apertures Ztl. The sleeve 23 is provided with an axial bore 14h within which test gauges (such as strain gauges or the like) may be received. Within the passageway 22 there is provided an annular rib 24 extending radially inwardly and forming a weir. It will be noted 19 and the output apertures Ztl but nearer to the output apertures.

The input port 17 is connected to a source of pressurized liquid by means of a conduit 25. The liquid passing through the conduit 25 is maintained at a constant pressure value by means of a pressure regulating valve generally indicated 26, disposed in a conduit 25a connected in parallel with conduit 25, so that pressure of the liquid iiowing into the input port may be maintained constant regardless of variations in the value of the liquid at the source. If desired, .a metering orifice 25h may be provided in the conduit 25.

The regulating valve 26 may be of any desired type, for example, as shown, it may have a housing .27 provided with a drain connection 27a and an inlet port 28 covered by a cup valve member 29 and maintained in biased contact therewith by a spring 30. The bias on the valve member 29 may be ymodified by means of a manually Aadinstable screw 31. Thus, when the valve 26 is set for a specic opening value, liquid in the conduit 25a at a pressure value higher than that for which the valve is set, will lift the valve 29 to bleed liquid from the conduit 25a and relieve the pressure in the conduit 25 downstream of orifice 25b until the pressure assumes the value for which the valve is set. Such bleed liquid is delivered to a drain through drain opening 27a. Also, should it be desirable to vary the pressure of the liquid to a higher or lower setting s that the cut-off speed of the overspeed sensing mechanism may be regulated, such variation may be effected by moving the screw downwardly or upwardly respectively, thereby increasing or decreasing the spring bias on the cup valve 29.

The output port 18 of the overspeed sensing device is connected to a pressure sensing mechanism 33 by means of a conduit 34. The pressure sensing mechanism 33 may be connected to any appropriate engine control, for example it may be connected to a servo-merchanism controlled throttle valve (not shown) which regulates the ow of motivating fluid to the turbine (not shown) the speed of which is to be regulated at the shaft 12. The servo-mechanism may be of the well known type which maintains the throttle in the open position as long as the liquid pressure therein is maintained at a high value and which moves the throttle to the olf position when the liquid pressure therein is reduced. The pressure sensing mechanism 33 comprises a housing 34a provided with a conduit 35 leading to the servtrrnechanism and a conduit 36 leading to a suitable drain. rlhe conduit 35 is in communication with a port 35a in the housing 34a which is adapted to be controlled by a piston member 37. When the piston 37 is in the upper position shown, the port 35a is blocked and the liquid in the servo mechanism is maintained at a high value, thereby maintaining the throttle in open position. When the piston 37 moves downwardly and opens the port 35a the liquid in the servo mechanism will flow through the conduit 35 into the pressure sensing mechanism 33 and thence to the drain conduit 36. The piston member 37 is slidably mounted in a bore 37a and has a port 37b disposed in an axial position and arranged to be closed by a reset valve member 38 extending through the housing. The reset valve member 38 has a portion of reduced diameter 39 thereby forming a shoulder 39a which is adapted to bear against a spot faced surface 40 which acts as a stop therefor against movement of the reset valve in an upward direction. The spring 41 acts to bias the reset valve upwardly against the stop 40 while the piston member 37 is biased downwardly by a spring 42 disposed within the housing and having one end in abutment with the surface 40. The piston divides the housing into a lower chamber 43 to which liquid is delivered from the overspeed sensing mechanism 10 by the conduit 34- and an upper chamber 44.

From the above it will be seen that when the value of the liquid in the chamber 43 is reduced below the biasing value of the spring 42 the piston 37 moves downwardly to uncover the port 35a while when the value of the liquid in the chamber 43 is suliciently high to overcome the force of the spring 42, the piston 37 will move upwardly to block the port 35a provided that the valve is in a position to block the port 37b.

In operation, as the engine shaft 12 rotates at a speed below the selected trip speed, liquid from the liquid supply source ows into the input port 17 of the overspeed sensing device 10, at a prescribed pressure determined by the setting of the regulating valve 26, thence through the input apertures 19 and along the passage 22, over the weir 24 to the output apertures 20, through the output apertures, the output port 13 and conduit 34 to the chamber 43 of the pressure sensing mechanism 33, thereby maintaining the piston 37 in the upper position to block the port 35a.

When the speed of the shaft 12 assumes a value higher than the selected speed, under decreased load conditions or the like, the impeller action of the apertures 19 tends to force the incoming liquid back into the input port 17, thereby preventing flow of liquid into the chamber 22. This unique feature is effected by the centrifugal forces created in the whirling liquid in the chamber 22 and the ports 19. It will be noted that the radial dimension, of the chamber 22 and the apertures 19 is suiciently large to create a whirling head on the liquid contained therein. The liquid trapped in the passage 22 by the weir will be thrown outwardly against the outside Wall of the passage, so that after the excess liquid contained therein goes over the weir 24 the remaining liquid is prevented from flowing into the conduit 34. In order that the impeller effect (suction effeet) created by the output apertures 20 will not be effective to draw liquid through the input ports 19 against the centrifugal lforces tending to throw it back into the port 17, a vent 14a to atmosphere is provided in the rotatable member 14. With this arrangement, as the liquid in the output apertures 20 is depleted, air will flow into the system to assume the space previously occupied by the liquid.

The interface between the rotatable member 14 and the stationary member 16 is necessarily a smooth sliding lit lpermitting free relative rotation of the two parts. This interface provides a space which serves as a leakage path allowing liquid in the port 18 to flow therethrough to drain, thereby reducing the pressure value of the liquid in the conduit 34 and concomitantly lubricating the overspeed sensing mechanism during shut-down. When such lower value is established, the piston 37 will move downwardly, thereby opening port 35a (while thc reset valve 38 is retained in the upper position by its spring bias to open the port 37b). The foregoing permits the liquid in the conduit 35 to ow through the upper chamber 44 to the drain through the conduit 36. and also to the drain through the port 37b and the conduit 34 thereby causing the servo-mechanism to move the throttle valve to the off position. The speed of the shaft 12 will thus be reduced until it falls to a value below the maximum selected value.

In order to cause the throttle to be restored to the open position, the pressure sensing mechanism 33 must be reset manually by moving the reset valve 38 downwardly against the bias of the spring 41 until it moves into Contact with the port 37b, whence it. may be released after a few seconds delay or sufficient time for the liquid in the overspeed sensing mechanism 10 to flow therethrough and into the chamber 43. When the value of the liquid in the chamber 43 is again attained, the piston 37 will again move upwardly to block the port 35a causing the pressure of the liquid in the servomechanism to be restored, thereby permitting the movement of the throttle to the open position again in the manner described previously.

Since as mentioned above, the interface between the relative rotating members 14 and 16 is such that. liquid may leak therethrough, it is desirable to provide an annular port 4S intermediate the input port 17 and the output port 16. This port may be connected to a drain by means of aconduit 46. With this arrangement, any incidental leakage along the interface from the input port 17 will be directed to the drain before it can flow into the output port 18 to affect the value of the liquid pressure therein. Such leakage along the interface to the output port would otherwise ladversely affect the sccuracy of the device.

A further feature resides in forming the piston 37 of the pressure sensing mechanism 33 with a lower portion 47 of reduced diameter, which portion serves as a stop and also serves as a mating surface with the lower surface' of the housing, so that when the piston 37 is in the lower position the port 37b is bloclced even though dan# ' germes the reset valve 38 has moved out of contact with the port. With this arrangement, when the liquid supply to the conduit 34 is cut olf due to excess speed of the shaft 12, the liquid in the conduit 34 is trapped and serves to provide some lubrication of the interface between the relatively rotating members 14 and 16 during the short transient overspeed condition.

Figs. 3 and 4 show a modification of the overspeed sensing mechanism shown in Figs. 1 and 2. In this modication, an overspeed sensing mechanism 110 is shown as formed integrally with an engine shaft 112. The overspeed sensing mechanism 110 operates in substantially the same manner as the overspeed mechanism 10 previously described and differs therefrom chiefly in the mechanical arrangement of its components. In this embodiment, the inner cylindrical portion 114 is integrally formed with the shaft 112 and, as shown, is provided with two axially extending bores 122 connecting a set of input apertures 119 with a set of output apertures 120 in somewhat the same manner as the annular passageway 22 of the first embodiment. The output apertures 120 are provided with sleeves 121 extending radially inwardly into the bores 122 to form circular weirs 124 surrounding the output apertures 120. The ends of the bores 122 are provided with cap members 123 which cap members are provided with vent passages 114a. A stationary member 116 is disposed in telescoping relation with the rotatable member 114 and is provided with an input port 117 and an output port 11S of annular shape and disposed in alignment with the set of input apertures 119 and the set of output apertures 120 respectively. v As before, the input port 117 is connected to a source of pressurized liquid by means of a conduit 125 while the output port 118 is connected to the pressure sensing mechanism 33 (shown in Fig. 1) by means of a conduit 134. Also, a drain port 145 may be provided in the stationary sleeve intermediate the input and the output'. ports and connected to a drain by means of a conduit 146.

In operation, the overspeed mechanism 110 operates iny the same manner as the overspeed sensing mechanism 10. That is, liquid flows into the bores 122, through the input apertures 119 and thence flows over the weirs 124, through the output apertures 120 and from the out put apertures to the conduit 134, to the pressure sensing mechanism 33 during rotation of the shaft 112 at speeds below the maximum speed setting. When the speed of the shaft reaches the selected speed, ow of liquid over the weirs will terminate because of the effect of the input apertures 119 on the incoming liquid. When such. l.

ow into the conduit 134 is terminated and the liquid within the bores 122 is depleted, atmospheric air will enter the bores through the vents 114a thereby preventing the suction effect of the output apertures from draw-- ing further liquid into the output ports 118.

As mentioned in connection with the first embodiment, the rotating member 114 may be provided with an axial,` bore 114b for the reception of strain gauges or the like, as well known in the art.

Referring to Figs. 5, 6 and 7 there is illustrated a fur-- ther modification of the overspeed sensing mechanism. shown in Figs. l and 2. In this modification, an over-Y speed sensing mechanism 210 is shown as formed integral-- ly with an engine shaft 212. The overspeed sensing:

mechanism 210 operates in substantially the same man i ner as the overspeed mechanism 10 previously described,. but differs therefrom chiefly in the means used to oppose the entry of the uid into the overspeed mechanism. upon the attainment of a selected speed. In this em bodiment, the inner cylindrical portion 214 is integrally f formed with the shaft 212 and, as shown, is providedv with two pairs of passageways 222 and 223, each extending axially, as hereinafter described. One pair of pas-I sageways 222 is constructed so. that in' normal operationy it connects a set of input apertures 219 with a set of;

6 output apertures 22) in somewhat the same manner as the annular passageway 22 of the first embodiment.

The input apertures 219 are provided with sleeves 219:1 removably connected to the cylindrical portion 214. When the sleeves are removed, a set of balls 220:1 may be inserted and disposed in substantially a radial line with the sleeve 219a and inwardly thereof in a seat 221 fromed in the inner surface of the passage 222 which is also the outer end of an interconnection 245a hereinafter described.

Similarly to the first embodiment a stationary member 216 is disposed in telescoping relation with the rotatable member 214 and is provided with an input port 217 and an output port 218 of annular shape and disposed in alignment with the set of input apertures 219 and the set of output apertures 220, respectively. As before, the input port 217 is connected to a source of pressurized liquid by a conduit 225 while the output port 218 is connected to the pressure sensing mechanism 233 by a conduit 234. Also a drain port 245 may be provided in the stationary sleeve intermediate the input and the output ports and connected to a drain by a conduit 246.

The passageways 223 are disposed inwardly of the passageways 222 and extend axially substantially the same distance as passageways 222 and parallel therewith. The set of interconnections 245a are aligned radially with the set of input apertures 219 and provide communications between passageways 222 and 223 which are closed by the balls 220a under normal operating conditions. The passages 223 are in communication with the drain connection 245 by means of a passage 245b, only one of which is shown in Fig. 7.

The overspeed mechanism 210 operates in substantially the same manner as the previously described overspeed mechanisms 10 and 110. The pressure of the liquid in the inlet line 225 is adjusted by a regulating valve 226, so that at normal operating speed the force exerted inwardly by the incoming liquid is such that the balls 220a are maintained in their most inward position; that is, each ball is seated in the corresponding groove 221 provided in the inner wall of passageways 223 so as to abut and close off the interconnections 245:1. The balls are small enough that while in the inward position the inlet apertures 219 are uncovered permitting liquid to enter the passage 222, the liquid then leaving through the outlet apertures 220.

When the speed of the shaft reaches a selected speed the outward force ou the balls due to centrifugal force (tending to unseat the balls) becomes greater than and overcomes the pressure force exerted inwardly by the incoming liquid (tending to seat the balls). When the balls move outwardly the flow of liquid through passageways 222 is at first restricted as the balls attempt to close oi the apertures 219 and in so doing simultaneously open the interconnections 245a communicating be tween the passageways 222 and the passageways 223. Upon the attainment of a sufficient centrifugal force the balls move to the most outward position, abutting the outer wall of the passageways 222, and in so doing close oi the input apertures 219.

Since the passageways 223 are connected to drain the unseating of the balls causes the iluid in passageways 222 to escape through the interconnections 24511, passageways 223 and 245b. The foregoing escape of liquid is in addition to the liquid which normally escapes along the interface between the stationary member 216 and the rotatable member 214 as described for the previous embodiments. The total effect of this leakage is that the pressure of the liquid in the line 234 is reduced and the piston 237 moves downwardly as described in the first embodiment and causes the servo mechanism to move the throttle valve to the olf position.

l When the ow into the conduit 234 is terminated and the liquid within the passages 222 is depleted, atmospheric air will enter through the drain connectiomvthereby preventing the suctiony effect of the output aperture from disturbing the position of Vthe ball;

It will now be seen that the invention provides a simple and reliable overspeed 'sensing mechanism which may be attached or formed integrally with the shaft 'of a rotatingT machine, the speed of which it isl desired to v control. Also, it will be seen that the invention pro-I vides a mechanism for effecting overspeed regulation entirely by a liquid system which is so arranged that in the event of failure of the system the engine is shut off, that is, the system has a built-in fail safe feature.

Although any desirable liquid may be used in the system, the use of oil is preferred since it is readily avail` able in engine and turbine installations and since it may inner and outer sleeves of the overspeed sensing mechanism.

Also, with the arrangement described, the amount of liquid used in the system is relatively small and the powerl taken from the engine shaft is of a very small order.

Although several embodiments of the invention have.

been shown, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various other changes and modifications without departing from the spirit thereof.

What is claimed is:

1. For use in a rotating machine having a throttle valve for admitting motivating fluid thereto, means for tripping said throttle in response to a reduced fluid pres-` sure, and a rotatable shaft adapted to be connected to a` variable load; the combination comprising an overspeed sensing mechanism having a rotatable cylindrical member adapted to be connected to a shaft, a stationary` member disposed in slidable abutting relation with said rotatable member and having a fluid input port and a fluid output port, said rotatable member having an input impeller and an output impeller in communication with said input port and said output port, respectively, passage" means in said rotatable member extending axially and' communicating with said input impeller and said output impeller, means for restricting the ow of inlet fluid through said input impeller, said restricting means including an element responsive to rotational speed and movable in a radical direction in response to said speed, said input port being connected to a source of fluid at substantially constant pressure and said output port being,k

adapted to be connected to a throttle tripping means.

2. An overspeed tripping mechanism comprising a rotatable shaft adapted to be connected to a variable load, a rotatable cylindrical member adapted to be connected to said shaft in a coaxial manner, a stationary sleeve in telescoping relation with said rotatable member and having a fluid input port and a fluid output port in spaced axial relation with each other, said rotatable member hav'- ing a first circumferential row of apertures providing av fluid input impeller and a second circumferential row ofl apertures providing a fluid output impeller, said input and output impellers being disposed in registry with said' input and output ports, respectively, said rotatable member having a first and a second axially extending chamber formed therein, said first mentioned chamber providing fluid communication between said input impeller andsaid. output impeller, interconnections communicating between said first and said second mentioned chambers, balls disposed within said first mentioned chamber and each being l5. be further utilized for lubricating the relatively rotating ,y

thatduring normal operation the force exerted on the ball.4

by the inlet fluid pressure overcomes the centrifugal force on the ball tending to displace the ball radially outwardly andsaid ball is `maintained seated thereby keeping said passage between said first mentioned chamber andy said second mentioned chamber closed but upon attaining a predetermined speed wherein the `centrifugal force on the ball becomes greater than the force exerted by the inlet liquid causing said ball to be displaced radially outwardly and open said first mentioned chamber to the atmosphere.

3. For use in a rotating machine having a throttle valve for admitting motivating fluid thereto, means for tripping said throttle in response to a reduced fluid pressure, and a rotatable shaft adapted to`.be connected to a variable load; the combination comprising an overspeed sensing mechanism having a rotatable ycylindrical member adapted to be connected to a shaft, a stationary member disposed in slidable abutting relation with said rotatable member and having a fluid input port and a lluid output port, said rotatable member having an input impeller and an output 'impeller in communication with said input port and said output port, respectively, passage means in said rotatable member extending axially and communicating with said input impeller and said output impeller, means for restricting the flow of inlet fluid 'through said input impeller, said restricting means including an element responsive to rotational speed and movable in a radial direction in response to said speed, said input port being connected to a source of fluid at substantially constant pressure and said output port being connected to a throttle tripping means; said stationary member comprising av sleeve in telescoping relation with said rotatable member, said input impeller comprising a first aperture in said rotatable' member andsaidoutput impeller comprising a second aperture in said rotatable member, said apertures being spaced axially from each other, second passage means, communicating means between said second mentioned passage and said rst mentioned passage, said ele- Y ment being adapted to restrict said communicating means,

movement of said element in a radially outwardly direct tion placing said first mentioned passage means and said second mentioned passage means vin communication with each other.

4. For use in a rotating machine having a throttle valve for admitting motivating fluid thereto, means for tripping said throttle in response to reduced fluid pressure, and a vrotatable shaft adapted tobe connected to a variable load; the combination comprising an overspeed sensing i mechanism having a rotatable cylindrical member adapted to be connected to a shaft, a stationary member disposed in slidable abutting relation with said rotatable member and having a llluid input port and a fluid output port,

said rotatable member having an input impeller and an t output impeller in communication with said input port and said output port, respectively, passage means in said t rotatable member extending axially and communicating with said input impeller and said output impeller, means forrestrcting the flow of inlet uid through said input impeller, sai'd restricting means being fixedly connected to said rotatable member, said input port being connected to a source of fluid at substantially constant pressure and said output port being adapted to be connected` to at throttle tripping means.

5. The structure-recited in claim 4 in which said re stricting means includes a member extending partially into said passage, the stationary member is a sleeve in telescoping relation with the rotatable member, the inputY impeller comprises a first aperture in the rotatable member and the output impeller comprises a second aperture in the rotatable member, said apertures being spaced axially from each other.

6.` For use in a rotating'machine having a throttle valve for admitting motivating fluid thereto, a rotatable shaft adapted to be connected to a variable load 'and an overspeedfsensing' mechanism having an output port;V the anvar/s3 combination of a throttle tripping device responsive to reduced uid pressure from an overspeed sensing mechanism comprising a housing for a pressure sensitive valve mechanism, a pressure responsive movable abutment dividing said housing into rst and second chambers, said first chamber having a port controlled by said movable abutment, said second chamber being in communication With the output port of an overspeed sensing mechanism, said movable abutment having a valve-controlled port extending therethrough and serving as a communication between said chambers, a raised central portion of reduced diameter on said movable abutment movable into abutment with a portion of said housing when the pressure in said second chamber is reduced, thereby cutting off flow of liquid from said second chamber to said first chamber.

7. An overspeed sensing mechanism comprising a rotatable shaft adapted to be connected to a variable load, a rotatable cylindrical member adapted to be connected to said shaft in a coaxial manner, a stationary sleeve in telescoping relation with said rotatable member and having an annular uid input port and an annular uid output port in spaced axial relation with each other, said rotatable member having a iirst circumferential row of apertures providing a uid input impeller and a second circumferential row of apertures providing a fluid output impeller, said input and output impellers being disposed in registry with said input and output ports, respectively, said rotatable member having an annular chamber formed therein providing an axially extending uid communication between said input and said output impellers, an annular rib connected to the outer wall of said chamber and extending radially inwardly therefrom, said rib being disposed intermediate said input and output impellers, means providing a vent from said chamber to the atmosphere, and said input port being connected to a source of uid at substantially constant pressure and said output port being connected to a pressure sensitive control mechanism.

8. An overspeed tripping mechanism comprising a rotatable shaft adapted to be connected to a variable load, a rotatable cylindrical member adapted to be connected to said shaft in a coaxial manner, a stationary sleeve in telescoping relation with said rotatable member and having a liuid input port and a uid output port in spaced axial relation with each other, said rotatable member having a first circumferential row of apertures providing a iluid input impeller and a second circumferential row of apertures providing a fluid output impeller, said input and output impellers being disposed in registry with said input and output ports, respectively, said rotatable member having a plurality of axially extending chambers formed therein providing uid communication between said input impeller and said output impeller, a tubular member provided in each aperture of said second row of apertures extending radially into its associated axial chamber to provide a Weir, means for venting said axial chambers to the atmosphere, and said input port being connected to a source of iluid at substantially constant pressure and said output port being connected to a pressure responsive valve.

9. For use in a rotating machine having a throttle valve for admitting motivating uid thereto, means for tripping said throttle in response to reduced Huid pressure, and a rotatable shaft adapted to be connected to a variable load; the combination comprising an overspeed sensing mechanism having a rotatable cylindrical member adapted to be connected to a shaft, a stationary member disposed in slidable abutting relation with said rotatable member and having a fluid input port and a uid output port, said rotatable member having an input impeller and an output impeller in communication with said input port and said output port, respectively, passage means in said rotatable member extending axially and communicating with said input impeller and said output impeller, means for restricting the ow of inlet uid through said input impeller, said input port being connected to a source of tluid at substantially constant pressure and said output port being connected to a throttle tripping means, means for adjusting the cut-off speed of said overspeed sensing mechanism including a pressure regulating valve interposed between the source of liquid and the input port, said pressure regulating valve being manually adjustable to vary the pressure of liquid admitted to the overspeed sensing mechanism.

References Cited in the file of this patent UNITED STATES PATENTS 1,771,720 Miller July 29, 1930 FOREIGN PATENTS 681,348 Great Britain Oct. 22, 1952 UNITED STATES PATENT OFFICE CERTIFICATE 0E CORRECTION Patent No. 2,877,783 March l7, 1959 Milton IVIo Hobbs f-ed 'that error' appears in bhe printed specification It is hereby certi quring correction and that bhe said Let-bers of the above numbered pa'tzen'J Te Paten-b should read as corrected below.

column. 6, line 8, for Hfromd read n formed w; column '7, lline 45,

for "Tadoal" Tead :radial (SEAL) Attest:

KARL H. AXLINE Attesting Ocer ROBERT C. WATSON Commissioner of Patents 

